Metastable isotopes

Nuclide. Each nuclide is identified by element name and the mass number A, equal to the sum of the numbers of protons Z and neutrons N in the nucleus. The m following the mass number (for example, Zn) indicates a metastable isotope. An asterisk preceding the mass number indicates that the radionuclide occurs in nature. Half-life. The following abbreviations for time units are employed y = years, d = days, h = hours, min = minutes, s = seconds, ms = milliseconds, and ns = nanoseconds. 

The half-life of a radioactive species is defined as the time it takes for the activity of the sample to drop by 50%. In this activity, you will investigate the decay of 137Bam, a metastable isotope of barium that undergoes gamma decay with a half-life of several minutes. 

The metal is radioactive and does not occur in nature, as the half-life of all isotopes is shorter than 5 million years. It is found in readily isol-able amounts in nuclear reactors. It is an effective "rust-preventer" for iron and steel in special applications. The metastable isotope "Tc has a half-life of only 6 hours and is therefore used as a gamma radiator in medicine (radiation therapy and diagnostics). Of very little commercial importance. 

Among the technetium isotopes, only " Tc, a pure emitter ( ,nax=0.29 McV) with a half-life of 213 000 a, can be obtained (Sect. 5.1) in amounts adequate for studying the technical applicability of the element and its compounds. However, the widespread use of technetium is necessarily restricted by its radioactivity. The application of the metastable isotope Tc in nuclear medicine has quite superior significance and will be described in detail in Part B. 

The isotope of technetium is actually a special isotope of Tc-99 called Tc-99m, where the m indicates a so-called metastable isotope. 

The radionuclide needs to be a gamma emitter (long-range ionizing radiation) that can be monitored from outside the patient s body. It should have a relatively short half-life to limit the radiation dose to the patient. The radionuclide most commonly used is technetium-99m, Tc. The m indicates that this is a metastable isotope, that is, that its half-life is considerably longer than most isotopes that undergo gamma decay. 

The m means that it is a metastable isotope. It is not completely stable, but it does have a somewhat longer half-life than would be expected.) Technetium is unusual in that it is one of only two elements lighter than lead (the other being promethium) that has no stable isotopes. Therefore, technetium has to be produced in nuclear reactors. Technetium is produced from molybdenum. Since Tc-99m has a half-life of only 6.0 hours, supplies must be produced continuously and then transported quickly to hospitals around the United States and Canada to be used before too much of it has decayed. 

A word of caution regarding the neutron-activation analysis of fission-produced iodine-129 is in order. When that isotope is subjected to a neutron flux, 61% of the activated atoms are generated as iodine-130m (9.0 m) (209). By waiting until the short-lived metastable isotope has decayed (about 10 half-lives) before beginning chemical operations on the sample, complications resulting from isomeric transition induced reactions are eliminated. 

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